Wheel Fitment HELP!!

One thing that helps is a "Mock-Up".
Take a look at the wheels on it now and see how much clearance you have around everything...THEN imagine it with a larger diameter, a greater or lesser offset, more width, etc.
If you have 4 inches from the tire to the leaf springs, you can figure that 3 inches of additional width can be fitted with an inch of clearance remaining. Same goes to the outer body. The rear is easy though...It merely goes up and down and does not steer.
For the front, do a full inspection of the clearance around the tires and rim edges with the tires facing straight ahead as well as full left and full right. It is MORE critical to get it right at the front because of the greater amount of obstacles to keep clear of: Tie rod end, front sway bar, front and rear edges of fenders as well.
ALL 8 1/4" axles were built using the 4.5 bolt circle BUT you have to measure yours. I just bought another 1970 Charger with an 8 1/4" axle. I can move the spring perches on it to use in an A-body but it is 3 inches wider than a stock Valiant axle.